The present invention relates generally to systems for controlling rotation of rotating structures and more particularly to a system for carrying out control so that the circumferential speed in rotation of a rotating structure such as a capstan of an apparatus such as, for example, a tape recorder or a video tape recorder will be constant.
As a system for controlling the rotation of a capstan for driving a magnetic tape and causing it to travel in a magnetic recording and/or reproducing apparatus such as, for example, a tape recorder or a video tape recorder, a system of the following description has generally been used in the prior art. This known system comprises a rotation pulse generator comprising a gear made of magnetic material and provided on the rotor shaft of a capstan motor for rotating the capstan and a detection head provided to face the peripheral surface of this gear and a control circuit for causing the generation of a trapezoidal wave in response to the rotation pulses from the pulse generator and phase comparing the trapezoidal wave with reference sampling pulses thereby to produce an output an error voltage. This output error voltage of the control circuit is applied to the capstan motor to control the same so that its rotation becomes constant.
This known system for controlling the rotational speed of a rotating structure operates merely to control the rotational speed of the capstan motor at a specific speed by averaging it over a number of revolutions so that the width of the pulses generated by the rotation pulse generator will become constant. Consequently, in the case where there is an eccentricity in the capstan, for example, in this known control system, the circumferential speed in rotation of the capstan cannot be controlled at a constant value. For this reason, control cannot be exercised to eliminate the rotational deviation in one revolution of the capstan, whereby it is not possible to attain control such that the travel of the magnetic tape will be at a precisely constant speed.